Neurological complications and other causes of morbidity were studied in 122 of 131 individuals (64 males, 67 females) with tuberous sclerosis, in a popululation in which its prevalence was 1/26,500. Seizures occurred in 78 per cent, beginning at less that one year of age in 69 per cent (in more males than females in both cases) and after age 16 in 4 per cent. More males than females also had infantile spasms and persistent seizures. Learning disorder occured in 53 per cent (also in more males), all with a history of seizures, and was strongly correlated with age at onset of seizures, type of seizure and outcome for seizure control. Of subjects with learning disorder, 85 per cent required supervision for daily living and 65 per cent had little or no language; 97 per cent were fully mobile. Hemiparesis had occurred in eight of the 131, giant cell astrocytomas in nine bilateral polycystic kidney disease in two, and haemorrhagic complication relating to renal angiomyolipomas in six.